Patellofemoral instability: classification and imaging.

Patellofemoral disorders must be approached through an appropriate process of diagnostic framing, performed using language that is, as far as possible, unequivocal and a validated and organic classification system. At present, the classification proposed by the Lyonnaise school, which fulfills these requirements, is the most complete. This classification divides patellofemoral disorders into three groups: objective patellar instability, potential patellar instability and painful patella syndrome. It also identifies three principal factors of instability: trochlear dysplasia, abnormal patellar height and pathological tibial tubercle-trochlear groove (TT-TG) distance. Imaging is crucial for correct classification and for identifying and measuring the principal factors of instability. Up to now, the emphasis has been placed on the contribution made by traditional diagnostic radiology and computed tomography. In recent years, however, growing attention has been paid to the use of magnetic resonance imaging in the assessment of the patellofemoral joint and in the study of factors of instability, even though there is still a need for validation of this approach before it can be routinely used in preoperative planning.